Chapter 6.2: Cellular Transport Mechanisms

Osmosis, Diffusion, Facilitated Diffusion, and Active Transport

Cells use various mechanisms to move substances across their membranes. Understanding these processes is essential for grasping how cells maintain homeostasis.

• Osmosis: The diffusion of water molecules across a selectively permeable membrane from an area of low solute concentration to high solute concentration.

• Diffusion: The passive movement of molecules from an area of higher concentration to an area of lower concentration.

• Facilitated Diffusion: Passive transport of substances across a membrane via transport proteins; does not require energy.

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP).

Primary vs. Secondary Active Transport:

• Primary Active Transport: Direct use of ATP to transport molecules (e.g., Na+/K+ ATPase pump).

• Secondary Active Transport: Uses the energy from the electrochemical gradient created by primary active transport to move other substances.

Key Proteins and Structures:

• Na+/K+ ATPase: A membrane protein that pumps 3 Na+ ions out and 2 K+ ions into the cell per ATP hydrolyzed.

• Aquaporins: Channel proteins that facilitate water movement across membranes.

• Carrier Proteins: Proteins that bind and transport specific substances across the membrane.

Does Passive Transport Require Energy? No, passive transport (diffusion, osmosis, facilitated diffusion) does not require cellular energy.

Example: The Na+/K+ ATPase is an example of primary active transport, crucial for maintaining cell potential.

Chapter 7: Cell Structure and Function

Prokaryotes vs. Eukaryotes

Cells are classified as prokaryotic or eukaryotic based on their structural features.

• Prokaryotes: Lack a nucleus and membrane-bound organelles; DNA is in the nucleoid region (e.g., Escherichia coli).

• Eukaryotes: Have a nucleus and membrane-bound organelles (e.g., animal, plant, fungal cells).

Comparison Table:

Organelles and Their Functions

• Ribosomes: Sites of protein synthesis; in eukaryotes, can be free or bound to the rough endoplasmic reticulum (RER).

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for delivery.

• Organelles with DNA: Mitochondria and chloroplasts contain their own DNA.

Endomembrane System: Includes the nuclear envelope, ER, Golgi apparatus, lysosomes, and vesicles; coordinates the synthesis and transport of cellular products.

Exo- and Endocytosis

Cells transport large molecules via vesicles in processes called exocytosis and endocytosis.

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell.

• Endocytosis: The cell engulfs external substances, forming vesicles; includes phagocytosis (solid particles) and pinocytosis (liquids).

Example: White blood cells use phagocytosis to engulf pathogens.

Chapters 9 and 10: Cellular Metabolism and Photosynthesis

Oxidation and Reduction (Redox Reactions)

Redox reactions are fundamental to cellular energy transformations.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Anabolic Pathways: Build complex molecules from simpler ones; require energy (endergonic).

• Catabolic Pathways: Break down complex molecules; release energy (exergonic).

Example Equation:

This equation represents the oxidation of glucose during cellular respiration.

Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose.

• Stages: Glycolysis, Krebs cycle (citric acid cycle), and oxidative phosphorylation (electron transport chain and chemiosmosis).

• ATP Production: Most ATP is produced during oxidative phosphorylation.

• Oxygen's Role: Final electron acceptor in the electron transport chain; forms water.

Equation for Cellular Respiration:

Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy.

• Light Reactions: Occur in the thylakoid membranes; convert light energy to chemical energy (ATP and NADPH).

• Calvin Cycle: Occurs in the stroma; uses ATP and NADPH to fix CO2 into glucose.

Equation for Photosynthesis:

Relationship to Respiration: Photosynthesis stores energy in glucose; respiration releases it for cellular work.

Example: Plants use photosynthesis to produce glucose, which can then be used in cellular respiration to generate ATP.